Commercially available, low cost calculator integrated circuits provide economical and reliable devices for making counters and timers. Because of their large numbers, four-function calculator circuits and associated displays can be obtained for much less cost than individual counter circuits and display devices.
Calculator circuits having an autosumming feature are the simplest to use in a counter circuit. The autosumming feature permits a number to be entered via the keyboard switches and each subsequent operation of the add (plus) key then causes the entered number to be added to the accumulated result which is continuously displayed. An example of a low cost, four-function calculator having the autosumming feature is type MM5736 (National Semiconductor Corp.).
If used as a counter, a keyboard is not required. Only three switches for clearing (resetting), start, and count are needed. In addition to the calculator circuit and switches, a display circuit is required. A type NSA1166 (National Semiconductor Corp.) is a commercially available unit suitable for use with the MM5736. Because the power output from the calculator chip is usually limited, a digit driver is used to provide the power needed to drive the display. For the devices already identified, a type DM75492 (National Semiconductor Corp.) is suitable. The connections required between the specific devices are shown in application notes. See also, for example, Electronic Design 26, Dec. 20, 1974, pp. 90-95, and Popular Electronics, Vol. 8, No. 5, Nov., 1975, pp. 88, 90-91.
The count switch can be replaced by an oscillator-driven switch. If the oscillator has a frequency of one second, the resulting circuit is a timer displaying accumulated seconds. A problem with a seconds counter is that users are accustomed to hours, minutes, and seconds notation. The calculator, on the other, uses only decimal numbers. When circuitry is added to convert the decimal output values to hours, minutes, and seconds notation, the result is a complex and expensive system that outweighs the economies originally achieved by using low cost, commercially available calculator devices.
The invention described herein is a circuit for converting a calculator timer to a minutes and seconds display using low cost, commercially available devices in an economical configuration whose cost magnitude is of the same order as the simple calculator timer circuit.